Solid-state imaging devices with higher resolution are used in many commercial applications especially camera and also for other light imaging uses. Such imaging devices typically comprise of CCD (charge coupled device) photo detector arrays with associated switching elements, and address (scan) and read out (data) lines. This CCD technology is matured so much that now days a million of pixels and surrounding circuitry can be fabricated using the CMOS (complimentary metal oxide semiconductor) technology. As todays CCD technology is based on silicon (Si)-technology, the detectable spectral ranges of CCD are limited to the wavelengths below 1 μm where Si exhibits absorption. Besides, CCD based imaging technique has also other shortcomings such as high efficiency response combined with high quantum efficiency over broad spectral ranges. This broad spectral detection is required in many applications. One of them is the free space laser communication where shorter (in visible ranges) and near infrared wavelengths is expected to be used. Photodiode array having broad spectral detection capability, disclosed in this invention, is expected to provide those features not available in today's CCD technology. With well designed of the array, appreciable resolution can also be achieved in photodiode array technology.
Photodiodes especially of p-i-n type have been studied extensively over the last decade for its application in optical communication. These photodiodes are for near infrared detection, especially the wavelength vicinity to 1310 and 1550 nm, where today's optical communication is dealt with. Now a day, the photodetector speed as high as 40 Gb/s, as described in the publication by Dutta et. al. in IEEE Journal of Lightwave Technology, vol. 20, pp. 2229-2238 (2002) is achieved. Photodetector having a quantum efficiency as close to 1, as described in the publication by Emsley et. al., in the IEEE J. Selective Topics in Quantum Electronics, vol. 8(4), pp. 948-955 (2002), is also available for optical communication. These photodiode uses InGaAs material as absorption material, and the diode is fabricated on the InP wafer. On the other hand, Si substrate is used for the photodiode for detection of visible radiation.
For covering broad spectral ranges, two photodiodes fabricated from Si and InP technology and discretely integrated, can be used. Monolithically, wafer bonding technology to bond Si and InP can be used to fabricated the photodiode covering the wavelengths from visible to near infrared. However, the reliability of wafer bonding over wide range of temperature is still an unsolved issue and a high-speed operation is not feasible with a wafer bonding approach. It is highly desirable to have a monolithic photodiode array, which could offer high bandwidth (GHz and above) combined with high quantum efficiency over a broad spectral ranges (<300 nm to 1700 nm and also to <300 nm to 2500 nm). For using especially in imaging purpose where CCD or Si based image sensor based device are used, the multicolor photodiode array with the possibility to rapidly and randomly address any pixel is also very much essential.
It is our objective to develop a monolithic photodiode array for broad spectral ranges covering from <300 nm to 1700 nm wavelength detection with having frequency response as high as 8 GHz and above bandwidth, and high quantum efficiency over 90% over the entire wavelength region.
It is also our objective to develop a monolithic photodiode array (or single detector) for broad spectral ranges covering from <300 nm to 2500 nm with having the frequency response as high as 8 GHz and above bandwidth and high quantum efficiency over >90% over entire wavelength region.
Our innovative approach utilizes surface incident type (either top- or bottom-illuminated type) photodiode structure having single absorption layer which can provide broad spectral response. The absorption layers will be designed to achieve the required quantum efficiency and also required speed. The photodiode can be used as the single element and also as the array form.
In array form, for example top-illuminated type photodetector array, metal line to connect each pixel separately to the outside contact pads is utilized for making possible to rapidly and randomly address any pixel independently. As each metal line usually needs to connect outside pads to inside photodiode pixel (also mentioned as element), the pitch and element size is limited by the width of the metal line and array number. For example, the element size (i.e. photodiode pixel size) and pitch can be made to 5 μm and 10 μm, respectively, for the array size of 25×25 and metal line of 1 μm. For the bottom incident type of structure, the large number of pixels, even over 1000×1000 array of photodiode elements can also be possible.
According to the current invention, photodiodes having broad spectral ranges (<300 to 1700 nm and also <300 nm to 2500 nm), high quantum efficiency (>90%), and high frequency response, can be fabricated using the single wafer. According to this invention, in the case of photodiode array, each array can also be operated independently. The manufacturing thereof is also simpler as compared with the prior art. Some applications include multicolor imaging applications such as for astronomical observation, communication etc.